The present invention relates to the field of delta modulated communications, and more particularly to a system for improving the performance of an adaptive delta modulator.
Digital transmission of speech signals has become increasingly important in communication applications. Standard digital transmission rates for voice channels are typically of 56 or 64 kbps. Techniques which lower the bit rate without a significant loss in speech quality increase the efficiency of the communications channel, and are thus highly desirable.
One of the known techniques for achieving this result is delta modulation. Bit rates as low as 19 kbs have yielded reasonable speech quality using linear delta modulation. However, the quality of the linear delta modulation at such low bit rates is not suitable for commercial telephone usage. Due to the autocorrelative properties of continuously sampled speech signals, the speech signal can be estimated at each sampling instant as a function of the past sample values. Briefly, a delta modulated signal is produced by subtracting the estimated signal sample from the incoming signal sample, and transmitting the error signal in the form of a plus or minus indication (1 or 0). In linear delta modulation, also known as constant DM, the estimated value is obtained from a constant amplitude impulse driving a linear filter. In adaptive modulation, also known as variable slope DM, the estimated value is obtained from a variable amplitude impulse driving a linear filter. The adaptivity of the step size is generally designed so that the error signal is smaller for small signals and larger for large signals.
Because of its fixed step size, the linear delta modulator performs inadequately for commercial telephone transmission purposes, due to its limited dynamic range. The adaptive delta modulators have been developed to improve the dynamic range. One such adaptive delta modulator is fully described in "A Strategy for Delta Modulation in Speech Reconstruction", by J. S. Su et al., COMSAT Technical Review, CTR 76/110, Fall 1976, Vol. 6, No. 2, pp. 339-355, the teachings of which are hereby incorporated by reference thereto.
The adaptive delta modulator (ADM) as described in the above article is illustrated in the dashed line portion of FIG. 1, the associated demodulator being illustrated in the dashed line of FIG. 2 as well. The input signal X(t) is compared to the reconstructed signal y(t) in subtractor 10 to produce error signal e(t). The difference or error signal e(t) is sampled at the sampling frequency f.sub.s through a bi-state quantizer 15. The resulting bit stream, B(nT), where T is the sampling period, is applied on the one hand to adaptive step size controller 20, and on the other hand to multiplier 25. The adaptive step size controller 20 observes the bit stream through a window of four to five bits, and determines the suitable step size Q to be used in the delta modulation process. The product output P from the multiplier is applied to feedback filter 30, the output of which is in turn delivered to the inverting input of subtractor 10. The details of the adaptive step size controller, and the characteristics of filter 30 are fully described in detail in the above cited article, and need not be further discussed herein.
With brief reference to FIG. 2, the receiving portion of the communications system employs the delta demodulator. The incoming bit stream B(nT) is applied on the one hand to adaptive step size controller 20', and on the other hand to multiplier 25', to produce signals Q and P, respectively. The signal P is applied to filter 30', which in turn provides an estimated signal to low pass filter 50. The output of low pass filter 50 provides the reconstructed signal output. Elements 20', 25', of FIG. 2, are essentially identical to the associated elements 20, 25, and 30 of FIG. 1. Additionally, the adaptive delta modulator and demodulator of FIGS. 1 and 2 may be implemented entirely digitally or may comprise a combination of digital and analog components.
Although the adaptive delta modulators have improved the dynamic range of the delta modulation process, the adaptive delta modulators known in the art either suffer from blurring of speech quality, or experience weakness in resistance to transmission errors.